The present invention generally relates to methods of producing bulk forms with controllable microstructures. The invention particularly relates to a large-strain extrusion machining process capable of directly producing continuous sheets or strips of iron-silicon alloys that have controlled microstructures, including controlled crystallographic textures.
Electrical steel or iron silicon alloy (Fe—Si) sheets have long been utilized for their magnetic and electrical properties, most notably for the high magnetic permeability and electrical resistivity. Commercial processing of Fe—Si sheets is commonly done through combinations of multi-step hot and cold rolling, and their magnetic properties are routinely varied by controlling aspects of the commercial processing to produce thin profile (thickness (t) of about 0.3 mm) sheets with different microstructural features, i.e., different crystallographic textures and grain sizes. As known in the art, crystallographic texture refers to the degree to which grain crystal axes are aligned within a material.
In general, two distinct types of sheets having vastly different structures and magnetic properties have been produced. Sheets processed primarily by hot rolling are classified as non-grain-oriented (NGO) due to a weak (near random) crystallographic texture, while the application of iterative cold rolling and high temperature annealing is used to develop a strong cube-on-edge (Goss) texture resulting in grain-oriented (GO) sheets.
Both material structure and composition control the magnetic properties of Fe—Si sheets. Aspects of the material structure considered to significantly affect magnetic properties are the grain size (d) and crystallographic texture, the combination of which predominantly influences the magnetic permeability (μ), coercivity (Hc), and hysteresis loss (W) properties.
The main objective in developing textures in Fe—Si sheets is the influence on the orientation of the easy magnetization directions, along <001>. As represented in image (a) of FIG. 1, in the case of an NGO sheet, the texture is weak with a (near) random orientation of the <001> directions both in and out of the sheet plane, resulting in a low degree of anisotropy in the magnetic properties such that performance is nearly independent of the applied field (H) direction. NGO sheets are thus most commonly utilized in electric motor applications, where the field is rotating continuously. In contrast, GO sheets (image (b) of FIG. 1) possess a strong Goss texture from secondary recrystallization, wherein one of the <001> directions lies along the sheet length (rolling direction, RD), leading to near ideal magnetic properties in this direction (i.e., high permeability with low coercivity and hysteresis loss). GO sheets are primarily used in power distribution transformers, where the easy magnetization direction can be configured mostly parallel to the magnetic flux direction. GO sheets also exhibit a significantly larger grain size (d>4 mm), by virtue of the secondary recrystallization, compared to NGO sheets (d about 150 μm), although this characteristic is considered to contribute less than the texture to benefiting magnetic properties. As a whole, the goal of texture development in Fe—Si sheets is to reduce the misorientation of the <001> directions with respect to the applied H field.
In addition to structure, composition also influences the magnetic properties of Fe—Si sheets. Alloying with silicon significantly enhances the intrinsic magnetic properties of iron by increasing the permeability, while reducing coercivity, magnetostriction, and core losses. Commercial rolling, however, is limited to producing Fe—Si sheets with a narrow silicon composition range of about 1 to 3.5 weight percent Si, even though it is well-known that higher Si content alloys have superior magnetic properties. In fact, a Si content of about 6.5 wt. % is considered to be a preferred composition for many magnetic applications. However, major decreases in workability arise in alloys containing greater than 3.5 wt. % Si. Such alloys are generally brittle and have a greatly increased likelihood of cracking during rolling, which prevents their cost-effective manufacturing in the form of sheets.
Many attempts have been made to manufacture electrical steel strips/sheets with higher silicon content, including casting, hot forging, sputter deposition, spray forming, direct powder rolling, and CVD (chemical vapor deposition) siliconizing. All but the CVD siliconizing have yet to enter the commercial sector with any success and none have been able to replace the current Fe—Si (3 wt. %) rolled sheets that are predominantly available, despite inherently improved properties with higher silicon content.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if a method were available for manufacturing Fe—Si sheets in a more efficient manner and, if desired, with a higher silicon composition than is currently possible by rolling processes.